Microwave powered discharge lamps in which a light emitting discharge contained in an envelope is maintained by microwave excitation without electrodes are known in the discharge lamp art. Generally the microwave lamp comprises a microwave cavity to which microwave energy is coupled, and a transparent discharge envelope mounted inside the cavity.
A known type of microwave electrodeless lamp, termed reflectorless, is characterized in that an optical reflector used to control light emissions is not a wall of the cavity but is a separate part. For example, the cavity may be a cylindrical cavity the length of which is about twice the diameter, and is composed of two cup-like members joined together at their open ends. The first member may be formed of a layer of aluminum and comprises a cylindrical wall, a flat wall closing one end of the cylinder, and an open end. The second member is formed of a mesh, such as a reinforced tungsten mesh, and comprises a cylindrical wall, either a flat or spherical piece of mesh capping one end of the cylinder and an open end, the edges of which are joined to the edges of the open end of the first member. The flat wall of the first member may be removably secured to the cylindrical wall by conventional means such as machine screws. This provides access to the cavity for removal of the envelope, etc. The first member is provided with one or more coupling slots which run axially with respect to the cylinder on the cylindrical wall. The slots are used for coupling energy from a microwave source to the cavity, and will be discussed below.
A reflector, the axis of symmetry of which is approximately coincident with the axis of the cavity, surrounds the cavity. The surface of the reflector may follow a simple geometric surface such as an ellipsoid or a paraboloid, and it may be comprised of a plurality of annular facets each of which is sized and oriented so as to direct the light reflected by it in a desired direction, or it may be otherwise shaped as known in the reflector art.
A bulb is located on the axis of the cavity toward the end of the mesh of the second member. The bulb is comprised of an envelope portion and a stem which is located along the axis of the cavity and is fastened to the flat wall of the first member. Since the light source is energized by microwaves, the light it produces is emitted with significant power in all directions. The bulb is located well within the mesh; consequently, in excess of half of the solid angle about the bulb corresponds to the mesh.
However, a second portion of the solid angle about the bulb corresponds to the first member of the cavity, and light directed toward the first member of the cavity is for all intents and purposes lost.